The present invention relates to a special effects circuit for use in electronic musical instruments preferably electronic organs. As the instrument player is engaged in playing, it is frequently desirable to automatically and variably attenuate or accentuate a portion of the audio frequency signal to provide a musically pleasing change in the output tone signal. This attenuation or accentuation of a portion of the frequency range produces a constantly but irregularly changing characteristic in the tone output signal.
In prior art devices for electronic organs, the instrument player could manually vary the resonance point of a filter which altered the frequency characteristics of a tone signal. An improvement on the manual operation was provided in U.S. Pat. No. 3,569,603 issued to Donald Kern and entitled: Moving Formant Band-Pass Amplifier For An Electronic Musical Instrument which included a circuit to provide a signal envelope to vary the resonance point of a filter in the identical manner each time that the circuit was actuated. This prior art device provided an output tone signal which is attenuated in the identical manner each time the circuit was actuated. Of course, audio frequency synthesizers such as those developed by Moog and others are well known. These devices alter the wave shape and frequency characteristics of audio frequency signals to produce different audio outputs.